This specification relates to a color image processing method and apparatus thereof and, more particularly, to a color image processing method and apparatus capable of converting a brightness image signal into a corresponding density image signal.
A basic processing performed by a conventional color image processing method adapted to a color image processing apparatus is that image signals of RGB (red, green, blue), brightness image signals, are converted into image signals of YMCBk (yellow, magenta, cyan, black), density image signals. In case of a halftone image, UCR (under color removal) and black color toner/ink densities are reduced, while UCR and black color toner/ink densities are increased as the density of an image becomes higher.
An example of the conversion characteristics of a halftone image in the conventional color image processing method is shown in FIG. 7. In FIG. 7, the horizontal axis represents values of YMC signals before the UCR processing and black color density extraction processing. The vertical axis represents values of YMCBk after the above processings. The black (Bk) component is obtained from the following equation: Bk= min (Y, M, C). In the case of a halftone image such as gray color, since each of the YMC values is the same, the black component value obtained from the above equation is equal to each of YMC values. Accordingly, Bk=Y=M=C is held on the horizontal axis.
In the UCR and black color toner injection processing, in accordance with the conversion characteristics shown in FIG. 7, since little Bk is output for a skin color dominant image in which the tonality is light, ideally, a pseudo-outline should not exist.
In the conventional method, as in electronic photography, since the screen angle causes reduction in resolution, colors lay on top of each other. In this case, three colors, Y, M, C, are accumulately overlaid to express a halftone color such as gray.
However, if the position to output the toners or inks of the three colors (Y, M, C) deviates, the color expressed can be reddish gray or yellowish gray (This is referred to as "color blurs").
Furthermore, when a gray color is spread to the entire copy paper, the output color can be partially uneven and it looks unnatural. This unevenness stands out when a homogeneous gray-color image is produced by a computer.
Images produced by a computer graphics technique have a tendency to have less skin color, more gray color, and high-chromaticity colors. Therefore, the problem in the conventional method is homogeneity of the color of the output image produced by color blurs.